Syntactic implication checks may use as_bdd() to compare Boolean
formulae.  By doing so, they register Boolean variables in an order
that is usially detrimental to the LTL translator.  The new,
clear_as_bdd_cache() function offers a mean to unregister these
variables, so that the LTL translator will register them again in the
a more natural way.

* src/ltlvisit/simplify.hh, src/ltlvisit/simplify.cc
(clear_as_bdd_cache): New function.
* src/tgbatest/ltl2tgba.cc, wrap/python/ajax/spot.in: Call it.